Torpor – Most bats have the ability to limit their use of energy when conditions are not good for finding food. When it is cool or rainy, insects might not fly. Windy conditions are difficult for both bats and their flying prey. When bats can’t get the food they need, especially smaller bats, they will go into something called torpor. Torpor allows a bat to drop its body temperature, lower its heartbeat and breathing thereby using less energy. Daily torpor is less extreme than that used for hibernation.
Bats look for a cooler place to go into torpor because that helps them maintain the lower body temperature needed. Bats in warmer areas often do not migrate or hibernate, but will use daily bouts of a milder torpor to deal with difficult conditions.
Hibernation – Many bats go through winter, an extended period of cold weather by hibernating. Winter limits insect availability (or may remove them altogether) so bats need to be able to survive difficult conditions without food. By hibernating, they can do that. Hibernation is an extreme form of torpor.
Bats feed a lot late in the summer to build a store of fat to get through the winter. However, since they are very small and must be able to fly, they are limited to how much fat they can store as a percentage of their body weight compared to a bear or other land animal before hibernating.
To get the most from their reserves, they change their metabolism a lot. Their body temperature can drop to just a little above freezing, their heartbeat to 5 beats a minute, and their breathing to once every 15 minutes or longer.
Waking from hibernation requires energy. Since bats are small animals, their energy reserves are small, too. Their normally wake a few, limited times during hibernation to clean out their system, pee and poop as needed, stretch and even fly a bit, then go back into hibernation mode. Their reserves can handle that energy expense. Forced arousals, wakening from disturbance, can cause bats to use up too much of their reserves so that they cannot survive the winter. People disturbing winter roosting sites and white nose syndrome are too big examples of forced arousals.
Migration – Not all bats hibernate. Some migrate to warmer locations. But migration is not just about warmth. Many bats migrate short to moderate distances to special caves that are best suited for hibernation during cold winters. Many bats in warmer states will migrate moderate distances from summer to winter locations.
Reproduction – Bats are very unusual for mammals their size…they have small numbers of young. Many bats have only one baby per year, others one to two, and just a few have as many as four.
Bats mate in the fall but give birth in the spring. Many females store the male’s sperm until spring, others keep the fertilized egg from growing until then. The advantage to fall mating has to do with the small size of bats. In the spring, they are coming out of winter at the end of hibernation, or at least, feeding less from at least some days of torpor. That means males are not as strong or healthy which would make mating behaviors more difficult. They avoid that problem by mating in the fall.
Then the females give birth in the spring when their food comes alive again. They need a lot of food to feed both themselves and their babies.
In many bat species, females come together to the same roost to stay as a group when giving birth and raising their young. This gives some added warmth for the group and young bats need more warmth to develop properly. In addition, it gives more protection from predators because the number of individuals can be distracting to many predators.
Babies grow to flying size within a couple of months, so by mid-summer or so, they start leaving the roost. At this time, the young are 95% the size of adults, so size cannot help you tell them apart.
Longevity – Also unusual for mammals their size, bats have relatively long lives. Most bats can live at least 10–14 years, and some have been recorded as living 30–40 years. No other mammal of their size lives that long.